The present invention relates to ported rotary kilns used, for example, in the reduction of iron ore, and more particularly to a butterfly valve actuation system for introducing air into the kiln at axially and circumferentially spaced locations along the kiln length.
One way to enhance the operation and increase the capacity of rotary kilns used in process plants is to inject air into the kiln from ports passing through the kiln shell. In this manner the pattern of the ports through the kiln shell can be designed to optimize the process. Depending on the process, air can be injected into the kiln above the product bed and the airflow is then shut off underneath the bed; or air can be added underneath the bed and airflow is then shut off above the product bed. In either case, there has to be a method to open and close butterfly valves mounted on a manifold attached to the kiln shell. The actuation of these valves also must be done in proper sequence.
The manner in which air is added to the kiln through ports passing through the kiln shell is typically accomplished as follows. A two-piece manifold surrounds the kiln. The outer segment of the manifold is stationary and is supported by structural members on each side. The inner segment of the manifold is attached directly to the kiln shell by leaf springs. The inner segment rotates with the kiln. A large rubber lip seal is attached to the inner, rotating segment of the manifold. The lip of the lip seal rides against the face of the outer, stationary segment of the manifold. Attached tangentially to the outer segment is a duct that connects to an air fan. Positioned on the face of the inner segment are pipe sections with end flanges arranged in a circle around the kiln. Sandwiched between each flange and pipe extending along the length of the kiln shell are butterfly valves. These valves are opened and closed in a specific sequence as they rotate with the inner segment of the manifold to control the timing and amount of air entering the kiln. Examples of such systems can be found in U.S. Pat. Nos. 3,794,483; 3,847,538; 3,945,624; 4,070,149; 4,208,181 and 4,209,292.
The interior of the kiln is heated to temperatures as high as 2,600xc2x0 F. The kiln shell is refractory lined to insulate the shell from the heat. However the kiln shell temperature can get as high as 700xc2x0 F. in some cases. Any valve actuation device has to rotate with the kiln, is subjected to high shell temperatures and harsh, dusty environments. Consequently it is difficult to use hydraulic, pneumatic or electric actuators in this application. The most reliable way to actuate the valves is through a type of a mechanical mechanism.
Using fully seated valves poses some design challenges in this application. Any type of a mechanical mechanism has to provide the necessary seating torque for the valve in a very limited distance. A typical butterfly valve with flexible seats has about a xc2xc-degree of rotation from the beginning of the disc seating to when the mechanical stop is reached. If the forces applied to the valve to create the opening and closing torque are applied beyond the limited seating travel of the valve disc, then parts of the valve itself and/or the valve actuation mechanism will be subjected to high stresses and the possibility of part failure or jamming of the valve exists.
In order to convey material along the length of a kiln, kilns are installed with a slight slope. Material is fed into the kiln by gravity and gravity causes the material to transverse the kiln as it rotates. As the kiln rotates, frictional effects of the material particles and interior surface of the kiln causes the material bed to take an angular position off center on the up turning side of the kiln. The center of gravity of the material bed is then displaced from the kiln centerline. As a result, the weight of the material bed applies a moment about the kiln centerline. If power is lost to the kiln drive, this material bed moment will cause the kiln to have a reverse rotation, referred to as xe2x80x9ckiln rollback.xe2x80x9d Under certain conditions there can be a build up of material that sticks to the inner surface of the kiln and is not symmetrical with the kiln. If power is lost when a material unbalance is at the top of the kiln, the kiln can rollback as much as 300 degrees. A valve actuation mechanism has to be able to have some means to accommodate this change in direction or rollback without damaging any valve or kiln parts or requiring re-setting of components.
In order to maximize the air porting and thus the airflow into a kiln, the piping arrangement used with a kiln can become quite congested. Therefore, it is beneficial to keep the space requirements of the valve actuation system to a minimum so that the number of ports can be maximized and optimally positioned based on process requirements.
The valve actuation mechanism must meet the following criteria:
1. The valves must open and close in a specific sequence and at specific points in kiln rotation.
2. The mechanism must produce the necessary torque to the valve stem to completely seat the valve.
3. The mechanism must not apply a torque to the valve stem greater than maximum value recommended by the valve manufacturer.
4. The assembly must have flexibility built into it to allow for installation misalignment and tolerance build-up of parts that could cause the mechanism to move beyond the limited travel required to seat the valve.
5. The assembly must be adjustable to allow for part wear, particularly the cam track.
6. There must be angular adjustment of the valve trippers relative to the kiln cross section that enables adjusting the points where the valves open and close.
7. The actuation assembly must be able to accommodate kiln rollback without damaging any components, altering the opening and closing sequence or requiring any re-setting.
8. Due to the congested piping assembly on the kiln, the valve actuation assembly must use a minimum of space.
In accordance with the present invention, there is provided an improved method and apparatus for supplying air to a ported rotary kiln. The improved method and apparatus involves the use of a mechanical butterfly valve and valve actuation system for introducing air into the kiln at axially and circumferentially spaced locations along the kiln length. The valve actuation system selectively and sequentially opens and closes the butterfly valves such that port air is introduced into the interior of the rotary kiln either above or beneath the product bed contained within the kiln as the kiln rotates. The valve actuation system of the present invention meets all of the criteria referred to above.
The valve actuation system of the present invention is incorporated with a butterfly valve comprising a valve housing having an air passageway including an air inlet and an air outlet, a valve member rotatable within the housing to open and close the passageway, and a valve stem defining an axis of rotation and connected to the valve member for rotation therewith. The valve actuation system includes an assembly mounted on the valve stem to rotate the valve member between its open and closed positions. The valve actuator assembly includes an actuator base connected to the valve stem for rotation therewith, a lever assembly mounted coaxially with the actuator base and valve stem to freely rotate relative to the actuator base and valve stem, and a lost motion mechanism interconnecting the actuator base and the lever assembly for transferring torque from the lever assembly to the actuator base as the lever assembly rotates. Preferably, the actuator base includes a hub positioned around the valve stem and a base plate projecting radially from the hub. Also, the lever assembly preferably includes a collar and a plurality of equi-angularly spaced lever arms projecting radially from the collar with each lever arm including a cam roller rotatably mounted on an axis parallel to and radially spaced from the axis of rotation of the valve stem. In the most preferred embodiment, there are four lever arms. Each disposed 90xc2x0 with respect to each other and about the collar.
The lost motion mechanism comprises a pair of spring assemblies extending between the actuator base and the lever assembly. Each spring assembly includes a swing bolt having a shank, a pivot end and a free end with the pivot end being pivotally mounted on the actuator base about an axis parallel to the axis of rotation of the valve stem, and the free end being received within an opening formed in an arm of the lever assembly. In addition, each spring assembly includes a coil spring surrounding the shank which is slightly compressed so as to provide a desired amount of resistance to the rotation of the lever assembly relative to the actuator base.
As the kiln rotates, one of the cam rollers on the lever assembly comes in contact with a stationary cam track referred to as a tripper. The rotation of the kiln then causes the actuator assembly to rotate as the cam roller travels around the tripper track causing the valve to open. At the point of kiln rotation where the valve is to be closed, a second stationary tripper closes the butterfly valve by reversing its rotation. By having two independent components to the actuation mechanism, i.e. the actuator base and lever assembly, connected to one another by springs that enable xe2x80x9clost motionxe2x80x9d to occur between the two components so that the lever assembly can rotate relative to the actuator base, sufficient flexibility can be built into the butterfly valve actuation system to accommodate the relatively small movement required to seal butterfly valves without over stressing the components thereof. This actuation system also applies sufficient torque to the valve to close it completely but further provides some allowance for misalignment, tolerances, wear and the like in parts. The present actuation system also accommodates kiln rollback without damaging any components, altering the opening and closing sequence, or requiring any resetting of the apparatus.
Various other features, objects and advantages of the invention will be made apparent from the description taken together with the drawings.